I don't think Ford continued using the stupid (but kinda cool) neon HMSL after '97. The regular Explorers used an array of LED's beginning in '98.
As far as I know, the '03 Sport Trac used the same HMSL as the '01 and '02 ST's, which used a regular bulb (#922 IIRC) in the red center and 2 more regular bulbs on the clear sides for the cargo light.
If the socket melted, perhaps the brake light was stuck on for a prolonged of time and overheated (due to a sticking brake light switch). If the side sockets melted, perhaps the cargo lights were left on.
Look on eBay for a used unit. I think, though I'm not positive, that the F150 P/U's used the same HMST/Cargo Light assembly. If you have access to a salvage yard, finding a replacement there would be the least expensive way to go.
Here's a link to a brand new one, but used one's go for about 1/2 the price and one from a salvage yard would be even less expensive.
http://www.ebay.com/itm/Ford-Explor...ash=item3f6355b752:g:3SAAAOSwrnNXP3Rt&vxp=mtr
Tip: Replace the regular bulbs with LED's. They will not generate enough heat to melt anything.