I noticed the other day that when I use the rear door wiper my voltmeter does a big dip down below 8 volts each time the wiper does a sweep across the glass. I've had a quick look and can't see any obvious sparks or wearing of insulation of the wires. It hasn't blown a fuse - as yet. Any ideas: is it a common fault, things to look at or check? Advice gratefully received. Thanks