! Hers I think has a fuel starvation problem of some kind. It’ll turn over but won’t start.
Jumped timing chain/belt
The last time I gave car repair advice, three different fire stations had to respond. Have you looked at those cheap leases with no money down? Your cars have probably reached the age where repairing them is just repairing a car that will have another problem real soon. I don’t watch a whole lot of TV, but it seems I saw some kind of lease deals for Volkswagen and Nissan. I don’t like to lease, but if you need reliable transportation for your wife, then a no-down lease with a good warranty might work for you. Or trade in one or both cars. Some guy was on FR claiming he got a no-down lease on a Chevy Volt for like $177 a month. Maybe it would make sense for you. Good luck.
My solution to car problems - a 1957 Bel Air and a 1967 camaro which I can trouble shoot and fix and don’t need smog.
How heavy is the keychain for your car that has the lock problem? A heavy keychain causes excess wear on both the key and the lock, to the point that one day it just stops working. This is usually first seen in the ignition though.
On the wife’s car, could be clogged fuel injectors. Could be debris in the tank or bad gas. Depending upon the age of the car, I’ve had a catalytic converter go bad causing excess back pressure in the exhaust, which caused the engine to die almost as soon as it was started, so it could be that too, I suppose.
Been decades since I’ve experienced any of that. Car trouble these days for me is invariably electronics related. Sensors, reflash ECU, that sort of thing.
Look underneath the car at the fuel tank. You will see where the fuel line goes into the tank, that is where the fuel pump is. Take a rubber mallet if you have one, your hand may work if you dont have a mallet, and bang gently on the tank around the pump, just jar it as good as possible. Lots of times this will cause the pump to work long enough to get it home or to the shop.
IF it cranks, it is the fuel pump and will have to be replaced.
As for the mercedes, is the switch such that can be easily removed? If so,unplug the switc, straight wire it and find a junk yard with a used switch to replace it with. Mine quit on a 1966 plymouth once and I had it running in less than 5 minutes.
I have a car from 1996 that suddenly wouldn’t do anything when I turned the key in the ignition. I also couldn’t open my trunk with the key. Turns out that my key was worn and finally didn’t fit correctly. I had to wait for my wife to return and used her very rarely used key. Things worked just fine so it was off to the hardware store to have duplicates made of her key...