Take a read of Justin's post regarding ASR issues and rebuilding the units.
Electronic Accelerator (EA) - Who's got the best price?
Many cars are affected by insulation breakdown and NOT the ETA units. Also, the ETA can often be rebuilt using new wiring looms.
http://v12uberalles.com/throttle_actuator_rewire.htm
http://pages.prodigy.net/jforgione/MB_asr.html
http://www.baxnet.net/merc/eta/ta-diags.htm
My issue resolved
ASR problem resolved - by 4 cable ties
These are the Key posts regarding ASR issues on S500s and SLs
Hope this helps
Lea