Now defrost light is blinking constantly is this a code

Hello, I'm Danny.

Yes, it is possible a code is now stored in your trucks computer. Are you sure when dash was put back in all the connectors/harnesses were plugged in and correct? If you don't have access to a code reader/scanner your local auto parts stores have a tool loaner program and will rent you one. Here is a tutorial showing what is involved checking codes: lets do a CAN scan which is the future of automotive repair you can get a CAN scanner from Amazon for about $50.00 here is a video to show you how.

https://youtu.be/u-4syLc-ifQ

I believe AutoZone will check codes for free if you drive vehicle to them. If you get back to us with code(s) we'll be happy to help diagnose your defrost issue. Hope this helps and thanks for using 2CarPros.
Danny-

Thank you for your help. I believe I've got it figured out. what I failed to mention I just remembered after the fact I change the heater control panel when I have the dash out because one speed on my fan control wasn't working. that control board must be bad because I put the old one back in and now it's fine. so thank you for your help. have a Merry Christmas.

However, the same speed that didn't work on the original board didn't work on the replacement board either. so there must be a problem somewhere else with that particular speed selection. I don't know if it's on the fan motor or the wiring. I might have to do some more investigating on that. I do know it's not on the on the control switch now though.

Hello again.

Be sure to check the blower motor resistor. Make sure it's not burnt/melted and has a good connection. Here is a link to testing wiring if it helps:

https://www.2carpros.com/articles/how-to-check-wiring

I've attached a picture of the HVAC wiring circuit below.Hope this helps and thanks again for using 2CarPros.
Danny-