I started my 89 325is up this morning to go to work and the usual "CHECK" light comes on in the dash. So I check the warning panel above the rear view mirror and everything is lit. I turn the car off. I turn the car on now the "CHECK" is on and only the Brake warning lights are lit. I turn the car off, surely this will clear everything I thought. I turn the car on "CHECK" light is on and the Brake Lights warning indicator is lit. Repeat a couple times, close garage door back-up, mash the brakes, they both work, lights too. Whew I can get to work. Now where to look for the problem??? Where does/would the return voltage from the brake lights return to to turn off the "CHECK" brake lights sensor? This isnt a vital fix but it bothering me here a 0245PST in the morning. Thanks in advance.

Assuming you checked the third brake light in the back window and it good, you might need a new brake light switch which is located under the dash.