I've never heard of a 1.5 din, however, I just installed a single din Alpine in my 1999 this weekend. You should be able to install it if they make a .5 din tray to go underneath or above the radio.
The Alpine should be able to use the factory Toyota brackets that are holding the factory unit in (which eliminates your need for a full "kit"). If they don't make a .5 din tray or plastic piece of some sort, then you'll have to get creative as to how to cover up the hole that will be left.
When you get the harness for the Toyota (They've been the same since the 80's), you will notice that it has two plugs to attach to the Toyota end.
When I got my factory radio out, I only saw 1 plug to use. Through some research, I realized you have to also remove the Toyota factory amp, which is silver and just behind the radio. It is only two screws, but then you will be able to plug the wiring harness in to the Toyota. Just saving you a phone call.