1974 450sl won't turn over (repeat from tech help)

[phango13]

My apologies if some of this sounds daft, I'm only a few months into owning this baby, and my mechanical DIY skills are fair to midling have worked primarily on motorcycles in the past...

So here's the deal--the starter motor runs and runs, but no sign of life from the engine...Engine was cold at the time, has been running swell otherwise...First time this has happened...

Been having some electrical difficulties (tail lites out, no dash lites, no clock) and have a broken antenna that runs the battery dead if I put a fuse in the #2 slot (so i've been leaving thet fuse pulled until i can fix the antenna)...but all other fuses are new as is the battery...again, starter motor runs and runs so i'm guessing there's plenty of juice...

One other wierd thing--over the past few days, I think always when the engine is warm, the starter motor would come on only after the passenger seatbelt was engaged--even had to dis-engage and re-engage it once to get a start (again, battery's been fine thru-out)

But I digress--what I'd like to do is some basic troubleshooting before I haul off and have her towed to my mechanic...Unfortuntely, she's stuck downtown so I'm trying to compile as much info as I can befire I head down there tomorrow and give it a go...

So far, I've asssembled the following plan of attack:

1. neutral switch--try starting in netral, shift from neutral to park

2. fire--check pulgs/cap/wires, pull wire to check for spark

3. fuel--spray starter fluid into intake manifold and if that works, work backwards up the fule line from there...

Again, I'm guessing that the solenid is okay because starter motor runs??

Any other suggestions??