Replaced the CPS. Turned over for a second then died out.

Pep Boys had one for $85 I called Auto Zone across the street and they told me they didnt have the CPS for the crossfire and had no access to it. I asked for the 2004 SLK320 and of course they have it in stock for $55. I also picked up a set of torx sockets.

Took me about an hour to change it out. The most difficult part (after prying the harness off) was my socket didnt fit between the shield and the sensor to get to the E8 bolt. I had to loosen the E10 bolts that hold that shield / bracket / thingy so I could swing it out of the way and gain access to the sensor's bolt.

Hands ripped raw and bloody, I'm back at square zero with mystery no start.
To recap:
A couple months ago the car mysteriously wouldn't start for about 15 minutes. Then fired up and ran fine for months.
A couple weeks ago I got a CEL. I bought a bluetooth OBDII transponder and downloaded an awesome app for my droid called Torque (Torque — OBD2 Performance and Diagnostics for your Vehicle) which read the p0140 code.
Last Saturday I got in the car and BAM!, Mystery no start. It DID turn over a couple times then immediately fade out and die.
I did a less than expert job soldering SOME of the RCM joints that look like the posts are protruding out of the solder. I could use a smaller iron and do the rest of the board. However I tested the relays with the RCM plugged in and they all seem to work!
I put a new battery in the car and noticed a bit of corrosion caused by the old battery. I cleaned up a corroded ground wire that bolts to the frame/wheel well just under the windshield wiper fluid reservoir. Fresh battery yielded not a single crank.

I'm almost the point where I have to call AAA and have it towed to the local shop.