Combo hvac unit adding common wire for wireless thermostat

enter image description hereI am trying to add a wireless thermostat for my parents. I’ve put several in, but this one I have an issue I have not been able to google for an answer. They have a new combo unit. I need to add a c wire for the thermostat. The unit is all electric. There is currently an unused blue wire at the thermostat and at the unit. My issue is at the unit, there is no terminal strip. There are 2 brown common wires wired together, and the red, white, yellow and green are all together individually. I tried wiring the blue wire to the brown common wires, but that didn’t work. Where can I wire that blue wire so I can get the 24 volts that I need to run the thermostat. It would be simple if there was a terminal strip like normal. Thanks for any help