STOP TRYING BIGGER FUSES NOW ... 2 AMPS IS MORE THAN ENOUGH
All cb's have a reverse current protection diode fitted into them. This diode will allow power to flow through it and blow the fuse if the rig is inadvertantly wired the wrong way round. Once the diode has gone short circuit, it will blow any fuse in the power supply line.
Sadly, when this has happened, the audio amplifier ic usually gets damaged. I would say that by increasing the value of the fuses you have used will most certainly have caused at least the audio amp to die.
You have one slight possible chance that the short is in the cable, or connector in the back of the rig. Otherwise, you are looking at a repair which could be anything from a few pence (diode) to a few pounds (diode & audio amp) to a wrecked rig not worth repairing.
For a number of years, if I was asked to repair a rig which had been damaged by reverse polarity, I would wire in a small bridge rectifier in place of the diode. This would protect the rig from any accidental reverse polarity in it's future life and cost only a few pence extra. Particularly useful if I was lending any rigs to anyone who might get it wrong way round.
The fact that your rig is blowing fuses MIGHT mean that the diode is still short circuit (very unlikely,) if you are very very lucky this short will be protecting the rest of the unit, but if you finally blow it by increasing the rating of your fuses and it goes open circuit, then the power applied will be going through the rig and if you have it on the wrong polarity then your rig will have suffered considerable damage.