from footballgroundguide.com:

The ground can be seen from the M6, if you are coming from the North side of Birmingham. Leave the M6 at Junction 6 and take the slip road sign posted Birmingham (NE). At the roundabout below the motorway, turn right (the fourth exit), towards City Centre/Aston/Star City, Villa Park is well signposted from here. However to be on the safe side, turn right along Lichfield Road, at the second set of traffic lights on to Aston Hall Road. This road will take you down to the ground. Mostly street parking (don't be surprised though if you are approached by kids wanting to 'mind your car'), although this is not as plentiful as it once was due to a local residents only parking scheme now in place in the streets around the Witton roundabout area. Street parking is also available in streets around Aston railway station (which you will pass if you continue along Lichfield Road towards the City Centre. Alternatively free parking is available, but a 15-20 minute walk away at Star City which is also well signposted.

Post Code for SAT NAV: B6 6HE

I have parked near the ground before just off the exit of the motorway, somewhere near Aston station, but that was years ago at an awful England-Holland friendly so not sure how the situation is now. My experience usually finds that a 15-20 minute walk away from the ground is well worth it and my guess for Villa Park is that would be the case there as well. Would have loved to be there but things like work interfere

It is a nightmare around there. Lots of people pretending they are officials on side roads and estates taking £.

Last year it took us hours to get away. Much worse than Selhurst. See you there!!...

Get off the M6 at Spaghetti Junction. DON'T get on the A38(M)...follow the signs for the A5127 (Lichield Road). Go down the ramp to the roundabout and then get on the A5127 heading towards Birmingham city centre (back under the Motorway.) About a quarter of a mile down the road, there's a public Parking Lot on your right. Park there. It may even be free after 6pm. Once parked, walk out onto Lichfield Road heading in the same direction you were driving and take the first right onto Aston Hall Road. It will take you more or less directly to the ground (you wlll see it in front of you). All in all about a 10-15 minute walk depending on how quick you are. This bypasses pretty much all match traffic as you park more or less right under the M6 and can get away quickly after the match. This is where I always park for Villa away.

Sydenham by birth. Selhurst by the Grace of God.