My A/C unit is leaking inside by the heating unit, I believe it is a clogged drain but if there are other things that would cause this please let me know.

Pull the hose off and blow through it then check where connects to see if there is anything
Inside the connection

1st and easiest try, is to suck it out with a wet/dry shop vac,stick drain into vac hose and then wrap some rags around the nozzel so all the suction is pulling through the drain

Use the shop vac from outside where the water comes out of the house. Then put some bleach in the drain at the inside unit pipe. Mine are PVC so it's easy to recognize them.