Where is the heater relay on a 1968 camaro?

If not on the heater housing under the right rear corner of the hood. Remove the inside glove box and find the wiring harness for the heater box and the heater resistor should be near there. One of the big red wire should be connected to the heater and resistor together.

yup he's right ya know.^

If you only have Hi speed, it can't be the blower relay. The blower relay only comes of for high speed, if you have high speed and no lower speeds, it can only be one thing, you have a bad heater resistor block.