you should be able to send signals to the BCM from a touch screen control using an Arduino the use a Arduino compatible relay board with an expansion board that holds four modules to control some of the things not controlled be the BCM, I saw a Arduino OBD2 project OBD2 project what they don't tell you is the smarter the car the more control OBD2 has over it, I don't know if the BCM is vin locked it would give you a good start over controlling every thing. the onstar is subscription and has collision sensors, engine failure monitoring. that thing is linked in like big brother. It can unlock the vehicle as it is connected to the BCM
ok go to the junk yard find one of the newer suburban's remove the body control module this is linked to locks and doors controls if you have the memory seat installed then it has a motor connection for that as well, and with out any one seeing you pull the sealing down and follow the wires back to the onstar module this has the GPS unit and grab the antenna for the unit it is that little bump on the roof on the right, then looking at the very pretty mirror remove the set screw do not yank it. there is a small plug on the back you need all of this. going to the grill in the center core support the external temperature sensor can be found. Last you need the inside temp sensor you can find this close to the air intakes in two places one at the rear ac unit on the right rear of the back cargo area and one in the front of the dash board.
be careful all this stuff will break easy make sure you have followed the wire back far enough to reconnect the parts.
The mirror harness is real important it goes down drivers side corner post to the fuse box assembly and off up under the dash to the right.
it also passes through the fire wall to the center core support. the mirror tell you inside temp and out side temp and the body control module controls the locks, and interfaces with the locks ,alarm front and rear ac unit.
don't bother with the radio it is locked to the ECM after 94 depending on the Canada or US mfg US is 94 and Canada is 95 and every thing after 95 is OBD2 serial bus digital interface including the instrument cluster and training control
small socket set metric and stand.
Security drivers with the hole in the middle you can get those at harbor freight for 9 bucks
Torx screw driver the same one the fits most heads lights today
Philips and straight screw driver
don't for get the flash light some of the places are hard to see
Oh the most very important part some good gloves the truck is not their because someone said they did not want it any more, it will have broken glass, twisted metal. you may have to look in a few.
look in the driver side door place for year and MFG location. you will notice things are slightly different.
This is all 12 volts and all digital only the high current output switched with relays, you will want to go under the hood on the drivers side and remove the relays you will know what ones to pull by looking in side the lid they are marked. take them while your there as those can be as much as 25 to 30 bucks each.