Might not be the cause, but the brake warning light can be activated by any dislocation in pressure between the two components of the dual braking system. That's why, if any brake work is done on these cars that involves opening the hydraulic system at the wheels (e.g. replacing a caliper, or even bleeding the brakes), it is first necessary to remove the brake warning sensor (switch) from the dual brake junction block (usually mounted on the fender beneath the master cylinder.

What happens is that there is a piston with a shoulder inside that block. If pressure suddenly drops on one side of the piston, it deflects to that side and the spring-loaded pin in the sensor drops off the shoulder, closing the circuit and putting the light on. To reset this, once both brake systems are once again closed, you merely unscrew the sensor. The piston moves back into center position and when you screw in the sensor the pin is once again depressed and the light circuit is opened (i.e. the light goes off.)

I seem to recall once having that pin shear off and having to repair one of these switches.

Having said this, if there really is a connection between the shift lever and the brake light going on, this probably isn't the cause of your problem. But it's so easy to check, I'd still try it first before having to try t decipher one of those wiring diagrams.

If it turns out to be electrical and not a safety problem and you can't solve it, use the Car Talk solution. Put tape over the light.