Updated Feb. 12, 2012
I added the truck version of the OHC to my 1998 RCSB GMT400. The regular cab trucks never came with the OHC and do not have any provisions for mounting. The roof is double skinned just like the extended cab, Tahoe and Suburbans. I added a front mount and added the two square holes to the inner skin so the tabs on the rear of the console mount like they were designed.
If you want to do this Mod you will need an OHC from the 1997-98 GMT400, and I believe the same console was used in the 1999-2003 GMT800. In the truck consoles the blank is removed and the DIC and lens fit right in. The mold is the same that was used for the S/T series so everything fits. This will NOT work with the Tahoe/Suburban OHC as that one had a larger area for the rear AC controls. However, if you have the Hoe/Sub OHC but do not have rear AC controls, then you can swap it out for the truck style. The mounting is the same.
As far as electronics, this DIC is from a 1998-2001 Olds Bravada. They used both the black box and 411 ECM so I have reason to believe that this mod will work with either ECM. I did not have to do any programming to make this work with the GMT400 black box data bus. It was literally plug and play. I did have to make my own harness to hook things up, but used the Bravada harness as a template. There are two different DIC's that were used, both will work. They were in the S/T series Blazer/Bravada/Jimmy. The Comp/Temp will have the US/MET, and On/Off buttons and the full featured DIC will have the US/MET, and Mode buttons.
Update: An install using a Blazer DIC module
What you need.
-DIC out of a blazer, a long console DIC has the compass built in
-wire of your choice (I used 6 conductor phone wire)
-I used 7 feet of wire from DIC to Radio I would suggest using about 8 feet
-9 feet of wire to the temp sensor
Ok here is how I did it.
I started by removing the screw on the front of the OHC so I could slide it forward to release it from the roof
Then unplugged the connector for the lights.
I then had to make my own wiring harness for it because I did not want to even try to get the factory one from the blazer.
If you make your own harness make sure you take notes as to what wire colors go where.
I soldered all of my connections but if you don't know how or are no good at it you can use crimp connectors, but you have to make sure your connections are flawless.
Then take the dash panel off so you have access to the radio wires, and remove your radio.
I ran the wire on top of the headliner and down the driver’s side A-pillar into the dash by the headlight switch under the cluster and then to the radio (most of your wires will be here.
The wires you need at the radio are as follows
Pin #6 ppl/white------>LED dimmer
#11 grey------>Illumination circuit power
#9 drk green---------> RAP fuse supply
#16 drk green/white ---------->VSS
I used the power and ground from the lights in the OHC.
The only wire needed out of the cab is the signal wire for the temp sensor. (lt. green/black)
I tested the DIC before the temp sensor and the class 2 serial wire was hooked up that’s why it reads (--)
I put a piece of wire loom over the temp sensor so it wasn't easily seen behind the grill
This is where I mounted the temp sensor using the factory clip from the blazer
And this is where you hook up to the class 2 serial data
Some finished pics.
Compass was not calibrated yet
I have not figured out why the range mode always reads Lo. It is probably because the 1996-1997 fuel tank sending unit sends the signal directly to the cluster gauge, not the PCM like the 1998+ fuel tank sending unit.
This is the only in operation pic I have