Dash vents not working, I think

Defrost is the default mode. Start checking for a vacuum leak. Start at the parking brake release diaphragm (I think yours is still vacuum operated).

How exactly do I go about checking the vacuum? Also what vacuum??? Sorry to sound like an idiot but these Devilles have a steep learning curve.

Get a length of vacuum hose the same size as is on the parking brake release diaphragm. Pull the hose off of it and replace it with the length you begged, borrowed, stole or bought. Now put a hand held vacuum pump on the open end and pump down a vacuum. See if the gauge on it holds vacuum. If the vacuum holds, the diaphragm is good. If vacuum falls, the diaphragm is bad. If you don't have a pump (not very expensive), just suck on the hose and plug it with your tongue when you have draw all the vacuum you can. The vacuum will hold it to you tongue. If it leaks down it will fall off. If the diaphragm tests good, then work your way back towards the supply at the firewall.

Wel I accidentally left a hard-wired stereo preamp on and killed my battery, I got it charged and I lost all the memory settings but now the AC was blowing through the front vents computer glitch maybe?

2000 and up SDVs and DTSs (or whatever they are called) have a ton of failures with the passenger side vent controls. I was in the shop of a Caddy dealer and saw this pile of devices on the bench. I asked what they were. Mechanic told me they were defective vent controls for late model Cadillacs.

Windows regulators on ALL GMs are also problematic. ($500 a pop).

For your intellectual crap, er scrap bag.

I found something interesting...

My dash vents works until the coolant temperature is at 190 degrees, then it automatically switches to the front window vents no matter is it's in ECON or AUTO mode with the AC on(it's 82 degrees today). If I switch it to warm air it stays at the window vents plus the bottom vents like it should. The AC works good and still cools down the interior but probably not as fast as coming from the dash vents.

This is not normal right?

Not normal.

So I guess my vent control went bad? Where is it located? Seems weird that it has to do with coolant temperature...

I think the ACM is back behind the glove box. Pretty expensive part as I recall so be sure it is bad before replacing it.

It's fairly chilly today(66~73 degrees) and the dash vents stayed on much longer than other days when it's hot out, so I'm thinking maybe this has something to do with a temperature sensor? Before I said it has something to do with engine temp but now I'm not sure(maybe outside/interior temp?), it's temperature-dependent though.