Q: I Reversed battery cables on My 1992 Mercedes SL500. Now it turns over but won't start

When the battery cables are reversed, It only takes an instant to do a lot of damage. The Alternator and the ignition modules are probably toast, And any or all of the ECU’s are probably damaged. The good news, if you can imagine any in this situation, is that this car was built before Mercedes started "marrying" control modules. That means you can do substitutions with a salvage car.if you can find one. Later Mercedes modules were matched to each other with a code so that chop shops could not get away with selling used modules but your car was built before this happened. Sorting this car out will take a technician skilled in electronics equipped with a lab scope. If you want to get some professional help with this, you can contact Your Mechanic to have a technician come out to your home or office to check out your car and give you an evaluation.