First off I just want to say that I'm new to the whole car PC thing, just in the planning process for my system, but I've spent a lot of time doing computer audio in my home studio and I work as a live sound engineer.
I just spent the last couple of days playing around with the techniques descrided in the PC tuning wiki and got it all working on my laptop system just to see if I could make it work before spending money on hardware for the truck.
I used a 1.6 pentium-m laptop with a firewire Focusrite Saffire pro 26i/o audio interface(probably not what I'd put in my car but its what I have to play with). I also tried to use a Digidesign Mbox but couldn't get it working(I've always found its ASIO drivers a bit weird with anything but protools software).
What worked for me was downloading this chinese ASIO driver for Winamp(had lots of crackling with the ASIO driver on wimamps website).
I routed its output to the SPDIF outs on the Focusrite then back into the SPDIF input so console would see it. I set my buffers pretty high too. I'd start with 1024 samples. The Focusrite uses milliseconds for their settings. I was using 2.0 firewire buffer, 40ms ASIO buffer. No pops or clicks, sounded great on my home studio monitors.
How are you routing audio from winamp into console? I had to loop through the SPDIF ins and outs to make it work.
Couple other things to check, sorry if I'm being to basic here, I don't mean to be condescending. Make sure the Motu, Winamp, and Console are all set for 44.1k sampling rate and that everything is getting its clock off your Motu.
Another issue that can be problem with firewire studio interfaces is the firewire chipset on your computer's firewire port. Many interfaces only work well with TI chipsets. Also you could try installing the firewire hotfix if you are running XP. There's alot of info out about this if you google it but here's a link to get you started.
Hope this helps you out.