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the light only comes on after reaching 35mph.....
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There is a good chance that the RWAL system is setting a code 10.
On Chevrolet trucks the code 10 is for the brake light switch circuit.
Beginning in August 1990, a software change was made in the ABS controller on all General Motors RWAL equipped vehicles. The change transforms codes 6, 9 and 10 into "soft" (non-latching) codes that only illuminate the warming light as long as the fault is present. If the fault is intermittent and no longer exists, the light will go out with the next ignition cycle.
For the reason in the paragraph above I don’t think you will find a trouble code without a scanner capable of reading RWAL brake codes. Few scanners can do this.
Code 10 – Brake lamp switch circuit. Check the adjustment of your brake light switch. This code can be set by “riding the brakes” but it can also be set be driving in heavy traffic when you have your foot on the brake for an extended distance. Basically the system is detecting the brake pedal being depressed when receiving a vehicle speed sensor signal above 35MPH for what it detects as being to long of a distance.
A failing brake light switch can cause the light to come on when the vehicle reaches 35 MPH.
As these trucks are getting older, this seems to be becoming a common problem. The brake lights can work the way they should, but
the brake switch, having more then one set of contacts in it, can cause problems with the RWAL system.
Even if no code 10 is stored in the system, the brake switch can cause the RWAL system to turn the dash brake light on when the truck reaches 35 MPH.
If you want to do more testing, the power comes to the brake switch from the “ECM-IGN” fuse on the pink wire with a black tracer. It should exit the switch on the purple wire. The power should go through the switch when the Brake pedal is NOT pushed (a Normally closed switch) all the way back to terminal F of the 6 wire connector of the brake control module, which is the terminal at one end of the connector with a Purple wire to it. With the key on, and the brake pedal NOT pushed, there should be power there.
Brake Switch adjustment
Here is how the switch is adjusted:
On the curved side of the switch, which is pointing down and somewhat towards the front of the vehicle when installed, the end of a lever protrudes slightly out of a slot.
While holding the brake pedal down a little ways, slide the lever that protrudes from the slot toward the driver’s seat.
Let go of the lever, then return the brake pedal to its fully released position, and the switch should self adjust.
Brake Switch mounting – Installing:
The body of the brake switch should not move with the pedal at all. When you have the switch removed look at the side of the switch that would be towards the parking brake pedal if the switch were still installed. There is a protrusion on the switch that must slide into an opening where the switch mounts. Somewhat like a big square lug into a slot.