If you pull off the driver's side A-pillar molding (one screw and just pull it off), the light green wire coming down the pillar is the backup lamp signal for the rearview mirror. You can tap in there or follow the harness down to the kick panel trip as suggested by kodiakz.
Pin 21 of the I/P cluster is the speed signal; however, the speed signal also goes to the radio connector C2, terminal E. Since you will have the radio out, this makes the speed signal easier to access.
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Black Sheep Hummer Squadron
(ME TOO)
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