15 inch speaker? sounds like a subwoofer.. and to power it your prolly going to need amplifier, along with a newer radio(tape deck or cd player).the stock one wont do it very good...
actually using a stock deck isn't too hard, its just usually easier to use an aftermarket one... my friend has a brand new '05 sunfire, and if you don't know, they have a larger size radio from the factory, i believe they are twice as tall as most aftermarket ones, and also it has part of the computer that controls some things, some people said it was just the door chimes, some said it has air bag stuff in it too so we "had to buy this $120 relocator part" which is supposed to relocate the foctory deck to the trunk so all these still work and we can put the aftermarket one in. anyway, what we did to set up his amp since we're poor is we went to futureshop... i think they're only in canada, but they own... or are owned by Best Buy, which i'm pretty sure is in the states, and we got this device (which should be at both stores) that takes a speaker input and makes it into an RCA output, which allows you to plug in the amp. i believe it was $50 canadian after taxes
next was the problem with the remote turn on wire, since all it needs is a +12V signal going into it to turn on the amp, it is easy to just tap into any accessory circut, this makes it so that the amp will only be turned on when the key is in the accessory, or the on position, i believe we used the windshield wiper circuit.
anyway, this worked out fine for us and that subwoofer could really pound, and still using the stock deck, so i would assume you can use this same technique on the stock opel am radio since all you need is at least 1 speaker output. oh, and also if you don't want to loose the speaker(s) then you just leave them connected and connect the speaker wire-RCA box in parallel.
hope that helped... and made sense, and yeah... i kinda do this stuff in my spare time