Okay, thanks to a couple of people on one of the Crutchfield forums I finally have some answers and a working receiver. Turns out the yellow with purple stripe wire is for amplifier power and should be connected to the remote power wire on the Pioneer receiver.
What is frustrating is that I posted on the D-90 Source forum, emailed Pioneer, emailed Land Rover North America and have no responses. I attempted to get assistance from Crutchfield tech support but spent more time proving that the unit was purchased there than getting help. After several emails to Crutchfield I was finally able to show that it was purchased there and only then did they tell me that they were unable to help via email or chat, that I would have to call during business hours, though they suggested I try their forum and FAQ sites.
RAVE shows the wiring but gives no hint of what the yellow with purple stripe wire is for. So, to help the rest of the community - should someone need it - here is the wiring information for the entertainment unit on at least one '97 Defender.
purple - battery positive, always hot
orange w/white stripe - ignition switched positive
black - ground
red w/black stripe - light switched positive
yellow w/purple stripe - amplifier positive, ignition switched
black w/green stripe - left rear positive
black w/blue stripe - left rear negative
black w/white stripe - right rear positive
black w/brown stripe - right rear negative
black w/yellow stripe - left front positive
black w/orange stripe - left front negative
black w/pink stripe - right front positive
black w/red stripe - right front negative