MB300e65 Have you tried moving the gear selector through its range while holding the key in the crank position? If this makes it crank, its the neutral safety switch.
Also try locking and unlocking each door including the tailgate one at a time then see if it will crank. If it does the alarm immobilizer may be acting up.
The solenoid/relay is on the starter, there is a 3 wire terminal in black plastic on the firewall that you can open and jumper the starter and start the car with the key in the run position. It is on the back of the wall that the brake booster mounts to and is a few inches toward the center of the car from the master cylinder. It is direct to the starter relay/solenoid.
See if this thread helps...try to follow Arthur Dalton's info. He was a very formidable contributor to this forum.
http://www.peachparts.com/shopforum/tech-help/246642-93-300e-wont-crank-need-help-diagnosing.html