Try Santiago for car radios, stereos, etc.
There's an area in Santiago on 27 Febrero @ 3 blocks past [south, I think] of the Palacio de Justicia, where there are 5 - 7 dealers & installers of car stereo stuff. The dealers sell the stuff; the installers are usually freelancers with their own equipment. Look around; they carry all major brands. 1 is Kaya Accessories, 27 de Febrero, Santiago, 581 3880. As usual, nothing has a price tag; if you like someting, negoatiate; if you don't like the 'final' price, walk away; then, come back in 15". Free advice: if you're going to spend more than @ US $100 - 200. for anything, try Santiago 1st. Prices on everything [w/o exception] are 30 - 40% lower than POP & Sosua. An hour away & 400 Pesos in gas; if enough money is at stake, well worth the trip. The problem with POP & Sosua: too many wealthy foreigners chasing all sorts of products who value convenience very highly.