Real similar problem I'm having with an 1992 SC 300. For about a month, the car wouldn't start on the first try. Leave the key in the on position for about 10 sec and it would start strong the next try. Initially thought it was the battery, but it didn't act exactly like it. Over the last thirty days, it got worse and worse until I needed a jump. Got a jump start and drove about 5 miles, BAM darkness. No Dash lights, no power steering, no power brakes, no engine. No nothing except vanity lights, seatbelt light and door going "ding, ding" when you open it. Got a new battery from Sears and took it to the car. Yes, you guessed it for the 10,000th time. Wrong polarity. Guy and Sears was a little questionable on the details and I was in too much of a hurry to check. Massive sparks when I tried to connect the terminals and after that my 120A Alternator Fuse was blown. I have no idea if that happened before or after I tried to put the wrong battery in the car.
Long story longer, I cannot get the 120A fuse out of the car to replace it. (Any ideas:)?) I have a new one, but I don't know if that is going to fix the problem. My plan was to replace the alternator if it didn't, but it sounds like that may not be the issue. I'll definately post if my situation gets resolved.