There are several good comments regarding installation and I thought I would add a few items.
The unit fit perfectly and works flawlessly in my 2006 AMG 55 SLK; Most of my comments will apply to all SLKs from same model years.
Allen provides a link to a video and for the most part, it is good. There are a few other ones out there. All have a few grey areas, but if you see them a few times, you will work through some issues. I will also address a few.
Torx 20 is the primary tool. You will also need a prying tool.
I read that all screws were not the same. I did not verify, but I segregated them using baggies. So I had a bag for ashtray/lighter screws, one for AC panel screws, one for head unit screws, and one for cupholder screws.
I have an automatic. You need it in park and the parking brake on so you can move the shifter plate back far enough to remove the ashtray.
You do not need to unplug anything except the old head unit. In fact, I recommend against it. I did not remove the cable from the negative terminal. All components can be moved to the side or far enough away to comfortably remove and replace the head.
By far, the longest and most tedious step will be the location of the GPS antenna. I read where someone put it near the center speaker but underneath. Didn?t make much sense until I removed the Cockpit vent unit. It?s located above the cup holder. It takes some persuasion, but the best method for me was to lift it from the bottom and rock it slowly to and fro while lifting more. There are three clips. Two metal clips are on the sides and a plastic clip is at the top. Eventually, the side clips will disengage. If you continue to lift, the top clip will also. I put the GPS antenna unit inside the vent. I ran the wire out from one of the openings and notched the area so the wire could pass and not affect the vent operation. My GPS gets excellent reception and the antenna is not visible. I struggled a little to reattach the unit until I realized there are three rails on each side and the center rail must align with guides on each side.
Allen was very helpful and if requested, he provides an operator?s manual. Good luck deciphering that. Overall, you will glean enough to set up your unit.