GMC Repair: 2000 Yukon XL 3/4 Ton air con. drain, yukon xl, drain line

Question
The drain line on air conditioner does not drain properly.  Service person has blown it out twice in 2 mths.  Suggested it is leaves blocking line, says its costly for him to take apart and clean.  I'm willing to do the job if I can see a manual or get instruction.  This vehicle has served me well.
Thanks.

Answer
to access the drain from inside the truck is very difficult as the entire dash assembly and carrier would need to be removed as well as the heater - a/c case, which would involve recovering the freon.  I can try to post all the steps but must warn you this is not an easy task and should not be attempted if you have never done it before