'99 SLK won't turn over but plenty of power.
 

It appears there is a relay switch to engage the starter. A few times, after the car has sat for about an hour, the starter would not engage. There is plenty of power to operate the retractable top up and down, but not a single noise from the starter. All other electrical devices also work. I resoldered the K40 relay based on a youtube video describing a similar condition. This condition does not happen every time and once the car has sat for 2 - 3 hours, it would start just fine. Any other suggestions? Thanks.

are the battery terminals clean and tight? check all connections, like at the starter. if tight, put a test wire to the switch side of the start solenoid while you're there. turn the engine over. if there's power to the solenoid then it's probably the solenoid or the starter, assuming there's power to the battery side. if there's no power, then probably the ignition switch or neutral safety switch.

Thanks, I'm getting a new starter. I can have the entire job done for under $200. I think it's worth it. Thanks again.

Try cleaning the ignition key (yep, the one you physically put in the ignition). My mothers car had a security no start issue... There was make-up, Cheeto crumbs, and other inner purse debris that actually caused the vehicle to not start. It also set the alarm off it you tried to manually open the trunk with it (not using the fob). She had a 2004 c-class that all this happened to. Windex and a microfiber towel.

Well, I replace the start and still the same issue. I didn't mind changing it for the car has over 150,000 miles on it and I plan to keep it for some time. Again, plenty of power, new battery and alternator. I have to wait over an hour to be able to start again. During that time, there is a 'single' click coming from somewhere in the front engine compartment. It is not coming from the starter. Any other ideas? Is there a relay controlling this somewhere. I have a PSE Pump on order. I don't think it has anything to do with the problem, but I needed one anyway. Let's see.

it's unusual for relays to go bad, but under the fuses there's a starter lock-out relay/cycle module to the fender side of the box. it's a multi-function module. could water get into your fuse bx? check wiring and connections in there, I've had mice get into my cars and raise hell, little buggers.