When I was in Canada over christmas, I installed an empeg in my brother's car and had similar constraints. I ended up with a small 75W-ish 4 channel amp from Walmart, of all places, which cost about $70 canadian. It was surprisingly good with the OEM speakers, giving sufficient volume for normal driving and nothing I could point at as bad sound.

I can't remember the make, but it was very small, about 6 by 8 by 2 inches at most, and anodised bright blue. I'd think that Wallyworld in the US would stock the same range as in Canada, so you may well be able to track it down. If you can't I can ask my brother what the make is.

Patrick