Hi moty30. While some people on this board are savvy enough to do this job wothout the removal of the dashboard, I have been not so fotunate in such a task. Howeve, I will say that it really is not as hard as you may think.
Your Spider was assembled around the heater core... You take the whole car apart until all that's left is the heater core!
Well, almost - remove the center console & the dash. Then you will have access to the heater unit. It is not as daunting as that sounds - you'll spend most of the time removing the center console. Then disconnect the main instrument panel & four bolts to remove the dash. Take lots of photos. That'll help you remember how to reassemble everything plus you can post them here - we likes photos.
Eric is right. Also, maybe write down step by step, each step so after you mount all beck together, you will just do it backwards. Use little ketchup paper holders and number them. Place screws inside as per step done. This will save you much time screwing up the screws in the wrongly sized holes. I did this 2 times, and hope not to do again EVER.