Why does my stop/horn fuse keep on blowing out?

Why do fuse's keep blowing ? A short circuit to (ground) somewhere . Do you know how to use a wiring diagram an a DVOM - digital volt ohm meter to test electrical circuit's ? Brake ? brake light's ? You can't get it out of park because of the shift lock solenoid ( shifter interlock system ) This is a safety system that won't let gear selector to move from park unless driver is pushing the brake pedal . The fuse blowing - no power to brake light switch , computer see's no signal so it won't energize the shift lock solenoid .
http://www.bbbind.com/free-tsb Enter vehicle info. Year , make , model an engine size . Under system click on lighting ,then under subsystem click on stop lamps . click the search button ,then the blue link.
Electric Testing Techniques You Need to Know
How to find short to ground blown fuse

What is the amp rating on the fuse that keeps blowing? I looked at wiring for stop lamps, 15 amp fuse. Unplug connector for stop lamp switch, see if the fuse still blows? Also see if anything at connector looks burned or melted.

You could have problem with fuse box, itself. Look at back side of fuse box, see if anything looks burned or melted.

you have what they call is a shorten earth wire has come off or maybe trailer wire has come off of the connector

The driver's Multiplex Control System is a small computer that controls (among many other things) the dash lights brightness, ignition key interlock, shift interlock, and has a tie in to the brake light system and horn relay. It may be that this small computer malfunctioned and started the whole problem chain.

However, I note from the factory shop manual that if polarity is reversed on the shift interlock--it can be damaged. Reverse polarity will ruin the diode inside the shift interlock. This could have happened when you jumped the battery--a brief period of getting the cables and terminals mixed up could have done it.

Checking the shift lock solenoid can be done by removing the steering column upper cover. Disconnect the shift lock solenoid connector (2 terminals). Using long jumper wires, apply battery voltage to terminal 1 (yellow wire side) of the solenoid and ground terminal 2 (green wire side). Check that the shift lever can now be moved out of Park. NOTE: applying voltage in the opposite direction of that above will damage the diode in the solenoid and will probably ruin it. Remove your jumpers and shift back to park and check that the solenoid locks. If the solenoid does not work properly, it will need to be replaced. This involves removing the end of the transmission shift cable and is a complex job. Remember, no power to the solenoid equals a locked solenoid--it takes power to unlock it.

The Multiplex Control Unit has a self diagnostic feature which checks the computer itself and inputs to the computer. The computer check is called mode 1 and the input test is mode 2. The diagnostic trouble codes are outputted to the ignition key light and beeper.

To test, first confirm that all fuses are good, especially fuse 9 (10A) in the driver's under dash fuse box (left kick panel) and fuse 13 (7.5A) in the passenger's under dash fuse box (right kick panel). Then use a jumper wire (a wire with small alligator clips on each end works best) to jump the two leads of the multiplex test connector. (This is a light green 2 terminal connector dangling from the same wire bundle as the brake pedal switch connector.) Buckle the driver's seat belt (avoids nuisance beeps). Now turn the ignition switch to on. After about 5 seconds, the ignition key light should come on and the beeper should beep for 2 seconds then stop for .2 sec. then beep once for .2 seconds. This indicates that the system is in diagnostic mode 1.

If no diagnostic trouble codes (DTC's) are present, the key light and beeper will not blink/beep again. If there are codes: about one second after entering mode 1, the key light and beeper will indicate the DTC and repeat it every 3 seconds. If there are more than one, the system will indicate them in ascending order. The codes will be a light flash and beep so you will have to count the flashes/beeps to keep track. The codes range from 1 to 6. Code 3 (three flashes/beeps) indicates a problem with the multiplex control unit itself. Write back here for other codes or for questions about mode 2.

replace the horn

by putting in the over size fuse you have melted some wires together and created more problems then you started with

A shift release solenoid is to blame causing this problem. It is shorted out and needs to be replaced

# 18 for radio and it is a 20amp...see below for all #'s got to and amps... 1-20amp-pwr outlet frt. 2-10amp-rear fog lights(export only)
3-15 amp-horn 4-10amp-hedlt low beam rt
5-10amp-hedlt low beam lft 6-20amp-bodycontrolmodule/pwr dr locks 7-spare 8-spare 9-10amp- rt pk lt /rt tail lt/trailer tow lt/cluster
10-spare 11-15 amp-flasher 12-15 amp-stop lts
13-10amp-bodycontrolmodule/cmtc/cluster 14-spare
15-10amp-pdc fuel pmp & auto shutdownrelays/skis
16-20amp-pwr outlet rear 17-15amp-rear wiper 18-20amp-radio choke relay 19-20amp-frt fog lts/trailer tow stop and turn lts
20-20amp-sunroof 21-spare 22-10amp-wiperswitch/frt and rear washer 23-10amp-lft park lt/lft tail lt 24-10amp-pdc blower motor relay 25-10amp-heatseatswitch/hvac controlhead/trailor tow battery charge 26-10amp-hedlt highbeam rt 27-10amp-hedlt highbeam lft
28-spare 29-15amp-heated seat module 30-10amp-heated mirrors/rearwindowdefrosterindicator 31-20amp-cigarlighter
32-10amp-radio/frtdoorlockswitches/bodycontrolmodule
33-10amp-skis/diagnostic connector 34-15amp-cluster/courtesy lts/radio/bodycontrolmodule/maplights/cmtc/underhood lt/vanity lts
35-spare 36-10amp-ignition run/start-orc/lft and rt siacm
37-10amp-ignition run only-orc 38-10amp-abs module/brake shift interlock solenoid 39-10amp-rear window defroster relay/flasher/backup lts

take it to a shop

There is normally a diagram printed onto the the inside of the fuse case.