Glass, well, the point is , it probably has a driver ~5 inches, and since the enclosure is plastic, will end up sounding a lot worse than your rears, and in fact will have less bass than the rears , considering the fact that
1) the driver is smaller 5" as opposed to 2 6x9 drivers
2) the enclosure will be smaller the volume of the plastic box as opposed to the whole boot/area under parcel shelf
3) will have to be driven off one channel of HU
4) plastic enclosure as opposed to wood.
It is not a proper sub , it would be more correct to call it a bass module. It helps supplement the mids and lows from the 3 inch full rangers you usually see on TV's. This is more like the woofer you see in cheap PC 2.1 systems.
A Bad analogy would be saying that you cant afford fog lamps , and if you could use the reflectors + bulbs from a torch instead
Do-able , but not much of a point to it