Well, it does have a P-Bus in (for CD changers). This means you could hook up the CD-RB10, which allows an AUX-in connection via the P-Bus. But, I'm not absolutely sure that it would work if you didn't have an AUX setting on the stereo. Perhaps it would work when you selected the CD changer setting?
I'd send Pioneer an email and ask them if your model supports the CD-RB10 P-Bus to AUX adapter. If so, the part is only like $30.
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Scott
MKIIa 10GB - 2.0b11 w/Hijack
MKIIa 60GB - 2.0 final w/Hijack