The Trapani salt flats are out in the middle of nowhere, and parking is right where you’ll be walking around, so it shouldn’t be an issue. Look up its location before going over because the salt farm can be hard to find. We plugged it into our GPS before going on the trip. Near the salt farm is a WWII bunker that you’ll pass by. To me, the salt farms at Marsala (about 12 miles from the Trapani one) are nicer, especially for photos. They too are out in the middle of nowhere and can be hard to find because the roads are poorly marked. There are two parking lots at the Marsala farms, very near where you’ll be walking. The only people at the farms are there to see them, or work there, so chances are slim that you’d have an issue. We didn’t. Parking at both salt farms is free, but there may be a guy (at Marsala) pretending he works there asking for a euro to watch your vehicle. He was funny and honest when we called him out, so we gave him a euro.
In Erice, there are a couple parking lots. One is by where all the busses pull in by where you catch the cable car. There’s another down the hill a little, and one in town by the church of San Giovanni Battista. Since the one by the church is right in town, I’d park there since more people walk by. Believe all the lots in Erice are pay lots, so have a few Euro coins as many of the machines don’t take the smaller coins. We parked at our hotel.