CLK55 reverse light wire tap reads 9V?

I'm trying to tap onto the reverse light to hook up a backup sensor. My multi tester reads only 9V on the wire I found. (Yes, I checked hot bus voltage and it IS 12V). I read in another forum that a computer determines voltage feed to certain systems. I DID change the bulbs from the std 1156 24W to 55 W halogens. Can't see a thing in the back of this car with the top up. Can anybody shed some light? (no pun)


Thanks!