Do you know that the serial port on the player is a DTE (i.e. it requires a null modem cable) and on the sled it is more a DCE (it needs a straight through canle)? I say more a DCE since the other signals available are DTE like.

Also you can't upgrade over the serial port using the standard upgrade software if the unit is in the car sled. The bootloader detects it and prevents you upgrading that way. You should be able to use Hijack and ftp the image across as long as you have dc_servers enabled in the config file though.

Sounds like the problems given your most recent report.