I have the same issue, the front heater stopped working last Friday. I did some research (after Tesla quoted me the repair for 1700 euro on MARCH 15TH) and I share my plan with you:
1. Check first if the fuse is broken. If broken, then replace it. Fuse should be 14x51mm 40A 700V AC/DC; Tesla part code is 1019010-00-A
The BUSSMANN / EATON replacement costs 2 eur/pcs, you may need to by a pack of 10pcs (around 20euro), the code is here FWP-40A-14Fa
Position of the FUSEBOX (part number 1049678-00-H) that contain this Heater fuse is under windshield wipers - front trunk access.
Instructions to replace the fuse are attached here:
Model X 2017 ptcHeater - Google Drive
and here:
2. If no heat is flowing after fuse is replaced then you need to replace the front cabin ptc Heater GEN3 (code 1060432-00-E)
instructions here:
drive.google.com
and here:
Good luck!