sl500 weak points--wiring
Have the wiring harnesses carefully examined slit open the covering jacket and inspect the individual wires on the engine harness and the electronic throttle actuator. Both are failure points on some years, and both are >$1500 to replace.
It seems BOSCH made a lot of wiring harnesses (used in lots of cars with Bosch engine management systems) with biodegradable wiring insulation. Well, they do degrade alright--the individual wire covers crumble to dust, and when jiggled--short out, sometimes taking ultra expensive (>$3000 each!)computer boxes with them.