There used to be a great old thread on this, but I can not bring it up, and I think the photos on it were from Photobucket, so they got photophucketed
Anyhow you you need some good ELECTRONIC either liquid or paste flux, and some ELECTRONIC solder. DO NOT use plumbing AKA the stuff the hardware store or HD/ Lowe's sells.
SRA rosin paste flux is one option, easy, and that will last you a lifetime, even if you are only 12 not
https://www.ebay.com/itm/3874987513...YbDcg/yb+FA1OdFZxB9qbxoS7T|tkp:Bk9SR9bby-fZZA
Go to a welding store and get one of those toothbrush sized stainless brushes so you can scrub the **** out of it and get down to clean metal.
While you are at it, solder jumpers across from the springy contacts for the IVR to the board traces, and loosen / tighten the gauge stud nuts a few times to clean/ scrub them, and clean the contact area on the pc board for the lamps, as well as the tips of the lamp socket spring terminals. Bend them if necessary for better contact.
On my old cluster, I elected to abandon the original harness connectors, as they were broken, etc, and just soldered wires to the pc board, brought out to a pair of Molex connectors, the mates of which were installed on the harness.