My 1994 5.0 mustang doenst want to idle and dies when u put it in gear. any ideas?

What engine does yours have ? 3.8 or 4.6 Do you have a professional type scanner that will show sensor data parameters ? Is the check engine light on ,or anyother light's ? Wrench symbol .
012 Idle Speed Problems Idle Speed Wrong I know yours only dies when put into gear , idles fine in park an neutral . Just showing what scan data is , if your not familiar with it. With the problem your having it's a necessity to have a scan tool that can view data pid's . You could spend a lot of buck's just replacing part's on recommendations from people on here . Testing an not guessing is the way to find the problem .

CK WIRING HARNESS FOR SHOT TO GROUND

Check the fuel filter. It might not be a bad idea to do a tune up.

check the timing and also if the idle jet is opperating correctly. cold start mec could also be faulty. you can program it out

buy a new battery

It could be your idle air bypass valve. They get sticky with age and don't let enough air around the throttle body to keep the car running when it's at idle.

Will it stay running if you keep your foot on the gas?

check fuel pressure -fuel filter/fuel pump--also check for vacuum leak or hose disconnected

have you been tuning it? look for the vacuum hose you knocked off.

if its in park or neutral can you rev the motor or does it stumble on itself?