if it only works on hi.. your failure is going to be found in the blower motor resistor.
really easy to get to and replace..
open the hood. on the firewall near the passenger side shock tower.
is a connector. it just pulls off.
the resistor bolts to the fire wall. should be 2 10mm bolts. or 8mm bolts.
they are cheapy purchased at parts stores. or one of our vendors here on the site.
i think rock auto has them for around 10$.
check this picture for help .
or join us on the facebook turbo dodge caravan and plymouth voyager group.