My ml350 1999 still wont start after replcing another radio or do i need to replace back the old radio

What make , model an year vehicle ? Engine size as well .

You don't have to replace it with a new one but you do need to place your old one back on, as it needs to be removed to replace your crank angle sensor.

Did it run before you replaced the battery? Do you have any fuel pressure? 10 year old fuel pump might have given it up now. See if there is fuel at the schrader valve on the fuel rail on top of the engine. Could be some anti-theft feature of the vehicle too, is it the right key for the vehicle?

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Bad starter or starter solenoid. Replace your battery if more than four years old also.

If you have swapped out your old factory radio with another factory radio from a different unit, it will not work. It is coded to the VIN#, Just to replace a radio at the dealer you must link up and download the current configuration from the old unit using the GM TECH-II, then remove the old unit and install the new one, upload the old config files to the new unit and hope it works. Sometimes it doesn't and you have to keep swapping units back and forth to download and upload and eventually maybe getting a successful upload. If all your doing is replacing the factory unit with another factory unit, it will work if you take it to a dealer and they do the process explained above. Just don't throw out your old radio, they will need it because all of your old config files are still in it!
I hope this info helps!~subyguru~

it is most likely that the radio was reset when your car lost all power. it is the radio's theft deterent system. your local dealer can provide you with a security code if you do not have instructions for reset in the owner's manual.

If you still have book it explains how to do master reset 'if not un plug from a/c and remove back up batt which should be under paper roll wait 5min before replcing batterys if over 2years old replace before they start leaking

check the door switch.or you may need to replce the main control panel. but you also need to check the thermal fuses inside the dryer cabinet,one of those can be bad.

Make sure that the Battery Terminals are Clean and free of Corosion. Then Put the Cables back on a Little Bit Tighter.

Look at the user manuals for both radios and compare the frequency assignment for each channel. find the ones that match and program the radios to talk to each other.

Patrick