why won't my heater get hot?

I am pretty sure my problem is with the vacume lines they are old! I have followed the ones I can see under the hood and replaced the cracked ones I found. Still cant get the heat on. I wondered if I can manualy operate the door for the Heat/cold its stuck on cold. Also where about that door is located. this might also help trace the bad line.
Any insight would help.
Thanks,
Angel