lol, couple of grams here and there all adds up!!
That said you could just take a piss... Or spit a few times before you get in the car !!
BTW, quality speaker cable DOES make a difference, maybe at low loads you won't tell the difference, but it removes a lot of the static and general noise you get at high volumes.
To wire a whole car it shouldn't cost more than £60 imo. Perhaps a few layers of skin on the knuckles too.
I'd say do it all together, set a few days for taking your dash and doors apart, and put everything in there.