Rpm`s spike up and down

I don't wanna laugh but the Vibration of the defrost is probaly wiggling a connection --most likely a harness ground on the engine bay firewall ---I'd check the grounds then the throttle position sensor connector connection and then the --Don't laugh distributor connections to inlude the engine to cab ground cable ---braided bare cable from the block to the firewall -

--the COIL pack may have a poor connection ---and I believe if correct on blazers the feed wire to the coil from the harness
is thick and pinkish that connection if loose describes the symptoms exactly

nothing works without a sufficient GOOD gound --loosen and retighten and ground wires-positive wires and connections ----GM is well known for wiring harness problems ---my 84 blazer
happened to share the same ground for the Blower motor and the distributor ---not anymore!
check your connections and those grounds --am I repeating myself ---and also continue to seek help in here if still persisting after that ---should only take 15 minutes to check it all
30 to fix most likely
hope this helps
HAWK

This is a pretty common issue for Trailblazers to have. Here's a link with more information about these GM gauge problems. And where you can purchase replacement parts on Amazon if you want to do the repair yourself. Trailblazer Speedometer