I have an 89 IROC and my speedometer has started "squeeking" and jumoing around. Can I simply oil the cable, or will it require a new one?

Also, how do I go about gaining access to the cable?

 

It's probably from the cable being dry, or gummed up from old oil. Getting to it from up top is a bit hard since it's buried in the dash. I would unscrew the cable housing from the trans, pull the cable out, clean it real well with carb cleaner, and lube it back up with a graphite based speedo cable lube (Autozone carries it). Then slide the cable back up into the housing, while turning it if needed. Get it to engage back into the speedo and hook back up to the trans. If it still jumps, then your speedo has the jitters.