@new_to_me901 the Duets are very well protected, therefore I personally doubt a simple failure of one element that is powered to V_IN as your cooler is would have created this. If the failure is in the wiring, I would search for some short that causes a higher potential to be inserted where it does not belong. For example, any ungrounded metal parts that carry more than one electric component might be relevant since they could act as a "wire" that connects these two components.
But without knowing the exact schematic of whatever you are doing with the Duet, it is impossible to say this for sure - especially since you seem to have some pretty nonstandard use case, with a cooler at a connection where a heater belongs and a heater at a place where a fan belongs... an ice-cream printer maybe? 😁
For testing, do the usual:
does each component work and draw a suitable amount of current if it is connected separately to a suitable power source?
Is the sum of all 3.3/5/12V currents within the range of what the Duet is capable to deliver?
can you measure any shorts between any two of the component's wires or any wire and the metal housing/frame of the component if there is any?