Well fortunately you don't have a lot of electronics in a car that old - I'm assuming your car is from the 1960s or early 70s.

I would start with the alternator and regulator. How much did you operate on the car before discovering the error - did you turn on the key or try to start it (that would be bad, the starter would have rotated backwards in that case).

Is there any obvious damage - did you let the smoke out of anything?

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech