On my 260 (with oem air con) I don't think there is a switch that would turn on the hot air vs the cold air. If you turn the fan switch all the way to the right, it should be blowing , no matter what the temp control switch is set to.
Are you saying if you have the temp control switch on cold, the fan will blow, but if you slide the temp control to hot, the fan turns off? If that's happening, I'm totally lost.
If the fan blows while the temp control is on hot or cold, that's normal to me. If your prob is not the fan, but just hot air vs cold air, then you may need to start checking your vacuum lines. that's what controls the heat being able to enter the cabin.
That's of course if your getting hot water to the core, for the fan to blow thru.
Remove the control panel in the dash and look behind it. There are several vacuum lines that hook to the selectors. Then trace them to where they go and see if any are missing/cracked etc.
Good luck!