Recheck the connections, you may have missed one, so its flapping about in the breeze.
check the wiring change the starter relay and clean ignition switch with electicial cleaner.
Since you changed your battery and starter I'm assuming you are getting fuel and spark is your problem. You should check your starter relay, the wires from the relay to your starter, the wires from your battery to the relay, the wires from your locking cylinder to the relay, and your locking cylinder. You might also check your alternator. You can take it to a parts store to have it bench tested for free.
check your wiring,also check to see if theres power leaving the relay.
Find the starter and follow the heavy gauge wire from the starter. It will go to the starter relay or battery. If it goes to the relay then follow the heavy wire from the relay which will go to the battery.
The starter relay is located on the battery side of the Mercedes 450SEL. It is close to the battery itself.
Follow the + battery cable from battery should connect to starter solenoid (relay)
It is a switch that temporarily connects the battery to the starter
Uh....Starter Relay?
Follow the + battery cable should connect to the starter solenoid (relay)
the starter relay is located on the rad support near the battery
Battery, starter, starter relay solonoid, or a charging problem i.e. alternator or voltage regulator causing battery to discharge. Or bad battery cable connection or dirty or loose battery cable connections can cause this problem. Need to investigate & narrow it down.
Postive battery cable should connect to the starter relay (solenoid)