My '99 started blowing hot air out of the vents. Hooked up the manifold and got normal/low on the low side and zero on the high side. Looks like the compressor clutch isn't engaging, checked the fuse and it's good. I can spin the clutch by hand and it moves freely.

Where should I start?

Could be your low pressure safety switch has failed, unplug it and jump it with a paperclip and see if the clutch engages, if it does then you know the switch is bad

Clutch engages, but no change in high side, low drops with RPMs.

Expansion valve?

if the clutch is not engaging, it is impossible for you to get pressure on the low side and no pressure on the high side. What should happen is you have the same pressure on both sides. System should be ''balanced'' since the clutch does not engage. To be clear, you should for example be reading 30 PSI on the low side and 30 psi on the high side, because the compressor did not compress.

at 20 celcius, you should have something like 70-80 psi in your system (both low and high side) if your clutch does not engage. You are probably lower than that, if it is the case, your clutch does not engage because you are low on refrigerant, which means there is a leak.

I use a cheap gas with dye and a cheap uv flashlight to find leaks.

I would guess that your coupler isn't depressing the schrader core on the high side. If you only have 30psi on the low side with the compressor not engaged, you're low on refrigerant, which means there's a leak somewhere.