No power after replacing the heater core? No dash lights. No interior lights, engine doesn't even try to start?

I was having heating issues in my van. I replaced the heater core and now the van has absolutely no power. I am talking no dash lights. No interior lights, engine doesn't even try to start. The only thing I have noticed is that the flashers flash super fast.
When installing the new core I unhooked the bottom of the doghouse, the brake pedal, the steering column, and I took off the plastic cover under the steering wheel where you could see the air bag. I did not see anything that was unhooked.
Does anyone have any ideas on what caused this?