I just had my turn signal replaced with an aftermarket turn signal combo switch purchased from 1aautoparts.com. The turn signal will not switch off after I make a turn. Would this be a problem with the installation or is it in the switch?

could be either.......

What causes the switch to turn off?

There should be a cam inside the switch as the wheel turns back to center it should trip the cam popping the switch back to the neutral position.

Then I suspect it was not the installation, rather it is the switch that is defective. Thanks for the reply.