heater fan sometimes does not work

1997 chevy blazer
about 90% of the time my heater and fan works fine. But about once a week I will turn on the heater and fan does not work. Usually it will kick on after about five minutes and sometimes does not kick on at all. Once it kicks on it does not short out or anything. You can feel the hot air barely comming out so I'm sure the heater itself is fine. What would cause the fan to only come on some of the time.

On four full-size Chevy products, K5 Blazer, two old pickups and a suburban, the problem has been in the big purple wire going from the relay to the blower motor itself. Contacts get dirty or corrode at either end. If your setup allows you easy access to this wire, try taking it off and putting it back on several times to renew the surfaces in both contacts. I am unsure of the way the newer blazers are wired, so this may be useful info only for the "great old ones."

I have the same problem. I also had the damper direction problem, but got that fixed. Now occaisionaly the blower doesn't come on when I start the AC. Since it's 104+ degrees here all summer it's on all the time.
Checked out all the vacuum problems, had selector switch replaced, but still it fails intermittently.

When it has failed and I leave the engine running, I wiggled all the connectors going into the heater box under the hood. Nothing happened. But when I knocked pretty hard on the top of the box, it comes on every time. Sometimes a slap on the top of the dash on the passenger side will make it come on.

Any ideas?

Forgot to say that this problem is on a 96 S10 Blazer