Have you seen this? It may help.....
http://www.germanbeerinstitute.com/Erntebier.html
Also, there is a brewery called Held-Brau which makes a Dunkel Bauernbier, but I could not find any info on German sites regarding how it's made.
I've also had good results with a false bottom / center pick-up tube combo. If I used pellet hops, I always included some whole leaf hops to help make a better filter bed.
I bought a new chest freezer in 1999 and use it for beer (dispensing temperature) and it's been running on an external controller (Fuji) in a basement location from day one and still working fine...... Burrp!
Can you post a pic and the model number of the A/C unit? What is your desired temperature inside the cabinet? If you're shooting for lager temp & it's relatively cool in the garage, it's possible that the refrigerant pressures / temps are are way below what it's designed for and not moving heat...
You may want to consider using strut (Uni-Strut or equivalent). Some of the big-box stores carry it in their electrical departments and sometimes you can find used stuff cheap at a scrapyard. It's easy to bolt together, has accessory hardware available and can be easily changed over the life of...