I have a C70, Personally the stock stereo needed to be up graded. I don't have the Dolby Surround that you speek of, with the center speaker. But I want to state that the Stock radio couldn't put out the power with out distortion. Well that's the situation I was in. I was getting rattles and just the off frequency. My fix to this was to rip out the stock radio (incredibly easy) and replace it with an EQ and a highfi head unit. With a little extra power added to all 8 speakers, I was able to obtain a noticable difference in sound quality. It's entierly a stock speaker system putting out nearly 50 watts a channel (RMS), no distortion, no rattles. If I may say the stock head unit is under rated for the cars speakers. Maybe that's why there's a rattle in yours?

But how to get access to your front speaker? I have had my under dash out, and knee plate off. if i where to guess it would have to be above the heater fan unit, and the only access to it is from the sides. you could try to poke around like that. other wise save the aggravation and go to the dealer.