Yeah, using existing wiring is fine. Foama is right about the charging though. You have a couple options. At minimum you need a diode. A solar panel wired directly to a battery will slowly drain it if it isn't in the sun. This diode will have to be rated for the full current your panel can put out. Number two is a proper charge controller which will charge your battery at max amperage until the correct voltage. These can be had fairly cheaply on Amazon or eBay. Its nice if you can find one with a overcharge or shunt feature. I forget what they call it, but basically once the battery is topped off, it'll divert what extra power it has to another load. This is handy if you want to use your excess power for something like... running your cabin fan in summer to automatically keep your car cooler.
As a side note I noticed just the other day that my alternator delete (charging at 15V when on) did make my battery loose a fair amount of water over summer. I'll be updating that thread shortly. I had to add water to it and it wasn't an insignificant amount either. Granted, it was charging at 50-80 amps when it was on, not 1.5.
__________________________________________
View my fuel log 2015 Mirage DE 1.2 manual: 47.2 mpg (US) ... 20.1 km/L ... 5.0 L/100 km ... 56.7 mpg (Imp)