I think your out of luck Mark.You need a multiplex for a 8-trk,and a 8-trk for a multiplex.Both of these are different from there basic counterparts.Looks like you will need to make that wire harness after all,shouln't be all that hard.
green=front fader for factory radio
black=power to radio
second connector:black top,brown top
blue,black bott,brown b,purp
black top=left spk.ft
brown top=left rear
black bottom=right front
brown bottom=left front
last connection of six: yellow,gray,ect.
these are for the convector(heat sink)
one last wireis the ground,should have a eyelet to fasten to floor under the console.
If you power your radio thru the 8-trk,dont hook up the green fader wire.Splice your outputs from your radio into the speaker plugs.Your radio will be shut off (automaticly) when the tape is inserted so you won't hear both units.