I'm at work, so all I can do is upload a picture I drew out a few months ago when I was throwing around some ideas - but this is basically how the system works. I think I've got like $100 into the compressor and $7 into the reducing tee from Grainger.

The previous owner had cut the air lines and run them to a schrader valve on the hitch, which would leak sometimes.. leaving me on the side of the road/in a parking lot reinflating it with a little 12v compressor! Ghetto.

Anyways, after figuring out where to mount it (the jack compartment, fits like a glove) I just had to run the power wire and air hose from the engine compartment to the compressor housing. I was too lazy to hack into the relay box to find the compressor supply wire, so I just ran the wire with the air line since they went to the same place (they're both protected in conduit.)

Pics to come later, but for all the haters (ahem, you know who you are) who said it wouldn't work and that I didn't understand the system... it worked perfectly first try!

 

So, a slight edit to the install (though not really necessary..)

I was outside playing with the SLS remote and noticed that there was a leak when the valve blocks opened up (though the up/down still functioned fine.. it was bugging me). The leak was on the short piece coming out of me "tee" fitting, going into the factory compressor. So I took off the threaded/compression compressor fitting, cut out the leaking park, and screwed it back in.

...and stripped the threads on the compressor side. ****. (it was plastic.. which is lame because it's a crush-type compression fitting for the air line!)

So in the interest of not being stuck at work, I somehow managed to find a spare solenoid valve for 1/8" NPT air lines, with resistance similar to the OEM valve blocks (since the SLABS ECU seems to know what is hooked up to it based on past experiments!) I just cut the wires going to the exhaust valve (the one attached to the compressor) and wired them to this new valve, and stuck the air line coming out of the reducing tee into my new solenoid instead of the compressor.

Works perfectly! It actually lowers/adjusts MUCH quicker now, except it's not exactly subtle when it lowers.. makes a *pshhhh* sound (and is no longer waterproof/submersible like the OEM solenoid valves, which I need to resolve). With the new exhaust valve, this system now totally bypasses the factory compressor/dump valve which I'm going to remove.