Hi,
I'm an electronics engineer by trade... I've been looking into putting my own design on the market - because from what I've heard this seller on ebay doesnt sell the best product. It's a rip off because all he is supplying is a relay, fuse, bit of cable & a few connectors!!! If I was going to market it, I would be including the H4 (headlight) connectors & to install it all it would take would be to attach the loom to the battery (+) & tap two wires from the headlight wiring loom...(of which I would clearly state colour etc). I'll be designing & making batches of looms soon, so keep an eye out!
Anyway, that aside - You will need some crimp terminals (Blue 2.5mm terminals - 6mm rings etc for the connection to the battery) , a crimp tool, wiring diagram (to identify the correct wires to tap into)... Not the clearest instructions on the world! What part do you need specific help on?
Andy.