"i'll try the wire from batt pos. to the red wire post at back of alternator.
should i disconnect the red wire from alternator"?

That is not necessary unless you plan to plug the new jumper wire onto the alternator.
If you do unplug it, just wrap it around something to keep it from contacting any metal. That existing red wire is (or is supposed to be) hot (battery voltage) whenever the ignition switch is turned on, so it's possible to short circuit it to the chassis ground.



"i've cleaned the connection. do i have to replace the instrument cluster and plug it in for this temporary fix"?

No.



steve