ABS & Break lamp light up

Hey, on my 1992 Astro Van the break & abs lamp is one when I start the car. The dealer has checked the breaks and its no problem with them, at least they cant find any problem. The van is an ex-taxi, and someone hooked the taximater to the wire thats goes from the right wheel to the abs-box. I have chekded the parkingbreak and the breakfluid sensor and they are both fine. The abs-system are not working, hence the light i guess. Are there anyway to get the abs-system to tell me whats worng ?