Do you mean the temperature controller probe (you said "thermometer probe")?

You can put it on the heat tape, you will need to raise the cage up enough for the probe to clear.

In summer, the temp of the cage floor will pretty much be the temp of the heat tape below it. In Winter, the cage floor may be cooler than the heat tape below it, so adjust your setpoint accordingly and measure the cage floor temperature with a second measuring device.