I've never heard of a P0500 error code. Did you check them yourself or did a garage do it for you? If you checked them yourself with a paperclip, the ECU can only flash digit-based codes at you, and AFAIK they don't go above two digits (I might be wrong though).
See this diagram: http://homepage.ntlworld.com/peter.betts/supra/TechTips/dsc_speedo_mph.jpg
Cutting pin 6 of the odo plug, blanking off the plug side of the cut and splicing the wire going into pin 5 of the speedo plug to the loom side of odo pin 6, will bypass the odo's output buffer. It's been done before with no ill effects, but of course you do this at your own risk. This assumes you don't have a speedo converter fitted. If you do, that changes the procedure a bit.
The pink wire you cut or spliced, could that have been the one going to the ECU (pin 2 on the diagram)? Any wiring alterations are prime candidates for things going wrong so it could be worthwhile you lifting the carpet up and having a look to see what you spliced into what.