AC Evaporator drain clogged

[DSS]

I'm pretty certain the AC evaporator drain is clogged on my 78 Chevy K10- When I use the AC for extended periods condensation (not coolant) begins to drip onto passenger floor. So not to sound stupid, but I can't find the actual drain to unclog it- Looked all over and under the evaporator case and no luck- Any suggestions on where it is hiding- Appreciate any suggestions (or pics) on how to find it - Thanks

 

[chengny]

It's right next to the seam formed by the evap casing and fan housing (on the evap side) and way up against the firewall:

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[DSS]

Found it- Thanks for the help!