Some cars where built with a safety switch on the brake pedal, it was a short run but they are out there. If it cranks exactly when the brake lights turn on, that would suggest something electrical is involved. If it cranks before or well after the brake lights turn on, then I'd be looking for something flexing and making intermittent contact. A couple of things come to mind. We had a '97 Dakota that Chrysler donated to our community college. It had an automatic transmission but to eliminate the need to design and build a totally different wiring harness for trucks with manual transmissions, they installed a jumper wire under the dash where it would have a clutch switch. For safety, you had to push the clutch pedal down to crank the engine. Since there's no clutch pedal with an automatic, they put that jumper in there in place of the switch. I built a lot of "bugs" that I could switch in and out for my students to practice diagnosing, and one of them was a stretched-out pin in that connector. Sometimes it would still work properly if you wiggled it a little. You might want to look under your dash to see if they use that same jumper on your car. In the Dakota, it was a dark green connector with two pins and a 4" yellow wire connected to those pins. On the assembly line someone plugged that connector into the clutch switch or that jumper, but since you don't have to worry about that, the connector and jumper can be cut off and thrown away, and the two wires can just be soldered together.
The next thing to look at is the electrical connector on the ignition switch. There was a lot of trouble with the contacts overheating in the switch, and that heat would migrate out to the connector where the terminals would get black from overheating and the plastic body would start to melt. There's multiple circuits in the switch and the one affected was almost always the heater fan / power window / radio / turn signal circuit, but if it is loose, I could see it flexing a little when you press the brake pedal and possibly affecting other circuits. Some electrical tests with a test light will help identify that before you tear the steering column covers apart. With the starter relay removed and your test light's clip lead grounded, you should find 12 volts on terminal 85 or 86 only when you turn the ignition switch to "crank". Sorry I can't remember which one. If you do, that entire circuit is working. If you don't see 12 volts, we have to work backward to find where it's being lost. Next, if you do find 12 volts on one of those two terminals, move the test light's ground clip to the battery POSITIVE post, then probe the other terminal, 85 or 86. The test light will light up if there's a complete (good) path to ground, and that is going through the neutral safety switch. If you do have a lit test light, it will go out when you shift to reverse or drive. It should only be lit up in park or neutral. If both of those circuits are working properly, the relay should be clicking when you turn the ignition switch.
A totally different way to test the circuit is to pop the cover off the relay and stick it back into the socket that way. Now you can watch the little flipper while a helper turns the ignition switch to "crank". If it trips, you know the entire low-current circuit is working. That includes the ignition switch, neutral safety switch, coil half of the relay, and all the wiring between them. If the flipper doesn't click, one of those two circuits isn't working.
Next, you can squeeze that flipper yourself. That will energize the medium-current circuit and send current to the starter solenoid. If the starter cranks, you know the relay contacts, solenoid coil, and wiring between them are good. Plus, since the solenoid energized the starter, you'll also know the entire high-current circuit including the battery cables and solenoid contacts are okay.
Friday, December 9th, 2011 AT 9:47 PM