Is it worth hooking up a 25 year old Rockford amp or am I wasting my time?

Still running a Punch from that era in my second truck and it's been fine for years. Those old Rockfords were built like bricks, way overbuilt compared to what you get new at the same price point. Stored in a garage they usally come back to life no problem. The heat is on the electrolytic caps inside, and honestly most of the time they're still in spec even after decades on a shelf. The oxidized fins are just cosmetic, that aluminum dulls out on every one of them that's been touched by fingers and air for 25 years. Before you bolt it to anything, test it on a bench or a known good battery with a fuse in the power lead, not wired into the truck. Set your gain all the way down, hook it to something cheap like an old door speaker, and power it up slow. You're listening for a hum, a pop through the speaker, or that relay not clicking in. If it powers up quiet and plays clean, it's a keeper. A little hiss at idle with the truck off and engine not running is normal on these, alternator whine is what you don't want. The other thing to check is the terminals themselves. Look inside the speaker and power terminals for green crud or a white powdery look, that's your real corrosion indicator, not the case scuffs. Scrape the mounting feet clean down to bare metal when you install it so the amp can ground through the chassis like it's supposed to. A bad ground will make any amp of this age act up and you'll blame the amp when it's the install. On 6x9s it'll barely break a sweat. Those things run cool at low loads, that's why the heatsink is so thick. And yeah, the end caps are the first thing to crack on a used one, so if yours are intact and not sun bleasted to powder, you did good at the swap meet. Worse case you're out a fuse and an afternoon.

I revived one of these a few summers back and the only surprise was how much dust had sifted through the ventilation slots, so a gentle vacuum through the openings before first power-up is a good habit. One thing I did on mine that nobody ever seems to talk about was measuring across the speaker outputs with a meter set to DC volts while it idled, since any meaningful reading there means the output stage is unhappy even if it sounds fine on a cheap speaker at low volume. Also, check which model you actually have before wiring the 6x9s, because a couple of the two channel amps from that era had a mono/stereo switch or bridging jumper that was easy to set wrong when someone had been inside it years before you.

I picked up one of those from a buddy who'd parked his truck for like six years. Only thing I had to do was swap the inline fuse because it had gone old and brittle and the holder was all rusted. Amp itself fired right up first try and I ran a pair of cheap coaxials off it for two years before selling the truck.

Only real thing I noticed on mine was the gains were set way up by whoever had it before, so first thing I did was back them off. Made a bigger difference than anything else. Plan on cleaning the contacts with a little contact cleaner before you push the wires in, old terminals that sat will oxidize right where the set screws grab.