What would be a good way to test this?
I mean other than hacking it into the wire harness for my car and seeing what blows up.

I have a fairly good multimeter. Dont know what most of the settings are for. I mostly use it for car/motorcycle and house stuff.
Can I use it to test my end product?

So far so good with the construction. Although my soldering leaves a lot to be desired.
Although considering I've never done it before, (except for plumbing) I think Ive done pretty good so far.