I have relay's fitted to my vans such as this one on ebay ...Item number: 400110004373 Great bits of kit and once fitted (2 years ago) I have never thought about the battery since, it works fantastic.
As for the streaks. I have a few jobs where the seals are often a problem, I wash then rinse as normal, then wait a few seconds and rinse again only the second time I only rinse to the glass edge and no futher. it usualy works fine.
The "wet cement look" sounds like paint or plastic oxidization leaching from the frame. do the same as for above and it should be fine.
Its all in the rinse.
