I haven't driven it in about five months and I can't get into it to jump the battery. the key won't open the door and the keyless remote doesn't work. How can I get into it?

Your key fob is fitted with an emergency key for problems like this. Turn the key fob over and look at the back cover. There will be a small square hole marked with an arrow. (See photo). Slide the small ridge in the hole in the direction of the arrow. The fob should separate revealing a normal key. Use this to unlock the door using the slot in the driver's side door handle.
Refit the key back into the fob BEFORE doing anything to the vehicle. Once open you will have access to the car to open the hood as normal.

1996 Roadmaster

1. Locate the programming connector located in the LH trunk. Trim may have to be pulled back to access programming connector. ( Connector has black/white wires leading up to it and it is not connected to anything.)
2. Jump programming connector terminals. Leave terminals jumped unitl step 6.
3. System should now lock and unlock doors to indicate programming mode has been entered.
4. Press any button on keyless remote once. System should now lock and unlock doors, indicating successful programming of keyless remote.
5. Repeat step 4 for additional remote or other existing keyless remote. NOTE: A maximum of only two Keyless Remotes can be programmed.
   
6. Remove programming connector jumper wire to exit programming mode.
7. Test remote(s).

My remote key won work

When Your Keyless Entry Doesn Work Enlighten Me

Does the spare remote work? Have you tried to reprogram it? These remotes are very cheaply made and prone to key pad failure. To program the remote click this LINK, if that doesn't work buy anew one on eBay for cheap.

You are in luck, a 2005 Ford Five Hundred Key Fob Remote is self programmable. Now these procedures can be a bit frustrating at times, so be patient, it's all about timing. If you get it programmed on the first try you are among a very few talented programming individuals! Also, if you need an affordable key fob replacement for your Ford Five Hundred, buy it here:
https://www.keylessentryremotefob.com/Ford-Five-Hundred-Remotes-and-Keys-s/5167.htm

Here are the programming instructions:

FORD REMOTE - THIS PROCEDURE RESETS ALL REMOTES FOR THE VEHICLE. ALL FUNCTIONING REMOTES WILL HAVE TO BE REPROGRAMMED DURING THE PROCEDURE There are a few different procedures that will program your remotes depending on what vehicle that you have. You can find your exact procedure in your owners manual. If you do not have your owners manual you can try any of the procedures below until you find the one that works for your vehicles
Make sure that you don't press the brake pedal at any time during or before the procedure.
Procedure A:
1.Close all doors, then open drivers door and press the Unlock button on driver's door while it is open, insert key into ignition.
2.Turn ignition from OFF to RUN 8 times within 8 seconds, with the 8th time ending in RUN. Door locks will cycle to confirm programming mode. (if the door locks do not cycle, then it was done incorrectly. You must start over by removing key and beginning from step 1)
(Off is the position where you can remove your key, RUN is the position just shy of starting your car.)
3.Within 10 seconds press any button on the first keyless remote to be programmed. Door locks will cycle to confirm programming. If you only have one remote then skip to step 5.
4.Within 10 seconds, press any button on the second keyless remote. Door locks will cycle again to confirm programming. Repeat step 4. for all other remotes.
5.Turn ignition to OFF. Locks will again cycle to indicate end of programming mode.
*If this does not work try the procedure with the door closed.
Procedure B:
1. Fasten seatbelt and close all doors
2. Turn the ignition from OFF to RUN 4 times within 3 seconds.
3. Turn the ignition OFF. The system will chime to confirm programming mode.
4. Within 10 seconds, press and hold any button on first keyless remote. The system will chime once to confirm programming.
5. Within 10 seconds, press and hold any button on second keyless remote. System will chime once to confirm programming.
6. Turn ignition to RUN to exit programming mode.
Please note: All steps should be performed in fairly rapid succession. If you never get the initial chime or door lock cycle, that means the vehicle has never entered programming mode. You should remove key from ignition, open and close the door and start over.

Have you tried to change the batteries again?

Either via an actual theft attempt or you (or someone with your keys) not realizing that you/they actually armed the vehicle and opened the door or trunk afterwards.

Essentially this sounds like the vehicle was entered after being armed and the proper precedures not met to reset it.

The car may just need to be locked from the outside with the key (not the remote or keyless entry) while the doors are closed and windows are up, and then also unlocked with the key...

However it sounds as though you have perhaps driven it since then. If this is the case

You may want to check all your fuses and replace the ones affected by the anti theft system.
Security - Pwr Locks - Horn - Pwr Trunk Release _ Int Lights - and maybe The Emergency Flasher as well.

Be sure and check under the Dash, Hood and under the Back seat.

Cadillac recommends after a theft attempt, the vehicle may need to be serviced. So I recommend going through the fuses first, even replacing the battery in your remote fob and then if the problems still persist. have the vehicle serviced.

Good Luck and Happy Motoring

Respectfully

Have you replaced the battery in the remote? Do you have another remote to try? If you haven't done so already that's where to start.

That's what happened to me when my battery died I opened the door with the key and tried to jump it and the Alarm goes off. then i put a new battery in the car and the problem was solved. Good luck

if its early 05 round heart shaped remote it needs programming into system at dealer with Tech2
later 05 to 07 had square (rectangle) shaped remote
1.2 ID Code Additional Registration
Note: This procedure will add additional remotes to the Keyless Entry Control Units memory. A total of 4 remote transmitters may be programmed into the Keyless Entry Control Units memory. When a total of registered and newly registered remotes exceed 4, the oldest one shall be erased.
Step Action
1 Note: Make sure that all the doors are closed and unlocked. Insert the ignition key into the ignition switch.
Open the driver's side door.
2 Turn the Ignition key to the “ON” position and then to the “LOCK” position three times
(The key should remain in the “LOCK” position)
Note: This step must be carried out within five seconds.
3 “CLOSE” and “OPEN” the driver’s doors once (Leaving the driver’s door in the “OPEN” position)
Note: This step must be carried out within five seconds after step 2 is completed.
4 Wait for 8 seconds (This wait time is critical)
5 Turn the Ignition key “ON” and then to the “LOCK” position once and turn key to the “ON” position (The key should remain in the “ON” position)
OR on/lock once
Note: This step must be carried out eight seconds after step 3 is completed.
6 “CLOSE” the driver’s door
The HAZARD lights should flash 3 times (This acknowledges the Keyless Entry Control Unit has entered programming mode)
7 Operate the lock or unlock button of the transmitters to be programmed; a flash of the HAZARD lights indicates the remote has been successfully programmed into the control unit
8 The HAZARD lights will flash approx 4 seconds after pressing the remote button to indicate the Keyless Entry Control Unit has exited programming mode.