I used the "sd_boxster modulator solution" a week ago and am very pleased with the results. Interestingly, I do not have the shutdown issue. I used a different modulator (although, in truth, it's probably the same but branded Alpine).
As for tapping into the speaker wires to bypass the OEM unit, if you don't have the Bose stereo it should be doable. . . but if you have Bose the existing speakers are 2 ohm and that causes problems with most aftermarket headunits.
Good luck!
TRawls