Water remains in the bottom of the dishwasher (18 months old). Disposal is clear, glass trap is clear. What should I look for?

First : if this started after a new disposal was installed then you need to check the Knock out on the disposal located inside the pipe that sticks out of the disposal where you conect your dishwasher drain hose. ((((( Common Problem )))))

Ok, moving on, your dishwasher has a independent drain pump. if you know the noise of the drain pump and you do not here it. its a good chance that its bad and you can disconect it and put a volt metert to the 2 wires and check for 110V. of coures after you have your volt meter in place and ready to go you will then put your diswasher in drain and check your meter.

Here is a breakdown of a dishwasher with a simialr model number. # 60 is the drain pump.

Water left in dishwasher