just get a decent 4 channel amp (25~30watt per channel). you may pull the speakers and take a look at the ohm rating. I know GM's Bose systems have whacked out ohm ratings. Some are in the neighborhood of 32 ohms. Don't have any experience w/ ford stuff. Make sure the amp has a built in crossover. This will go a long way if you can control the gain and the frequencies going to the speakers. You then can get that clean sound at high volume levels. Also check out the amps Thd rating. the lower the better..cleaner sound. You want to look at RMS ratings as well. Peak watts are just that, Peak.
I have an Alpine V12 amp in my suburban that was in it when I bought it. Seems to be a pretty high-end amp. I think it is around 75 watts per channel. I have JL audio 6.5" drivers in the rear door w/ crossovers and component JL's in the front doors. Some kind of Kenwood MP3 head unit. It thumps pretty good for only having 6.5" speakers, even better now that I tuned it. I believe the guy before had a sub and was filtering out all the lows. This will all change when I Daisy chain another Alpine amp w/ the 10" Infinity sub I just bought. They are on sale at Crutchfiled for 59 bucks. What to ya'll see my kickass sub box made from 3/4" solid core Red Oak Veneer Plywood. The joys and perks of working in the hardwood business.
To some up my advice:
Buy a 4 channel amp and speakers to replace the the OEMS. Speakers are cheap for the upgrade in sound vs. dollar