My E420 window, seats and radio work sometime then other time they just stop working. You can be driving listening to the radio and it just cuts off and on, same with the windows and seats. Anyone have an idea where to start?

Look in the fuse box and see if those are all on the same circuit / fuse. If yes, try cleaning and or replacing that fuse. Then clean electrical connections on that circuit - ground connections too.

If it stays off for a while, then you can use a multimeter along with factory electrical diagrams to track down where the problem is.

Also if both on the same fuse, you can use a multimeter to see if power is being lost at the fuse or not (problem further down the line).

If there was do-it-yourself wiring done to install the radio, then that is probably where the problem is. People will "twist" wires together and they later come loose (no crimp connector, wire nut, etc.).