Perhaps the reason why you are having trouble finding an insurer in Europe to insure your vehicle is because, pursuant to the laws of most European countries, the country of risk for motor vehicles is the country of registration.

So if your vehicle is registered in the US, the insurance risk will be considered to be located in the US, even though the vehicle is physically located in Europe. A European insurer is unlikely to be able to insure your vehicle because they cannot insure risks located in the US (unless they have the insurance licence to do so).

On the other hand, a US insurer is perhaps unwilling to insure your vehicle for use in Europe because US motor vehicle insurance policies tend to have a geographical limit.

Actually, it is a rather complicated situation and unless an insurer is experienced in cross-border transactions, it would not consider insuring your vehicle.

It is unlikely that you will be able to drive your car in Germany or Switzerland unless you have a valid third party liability insurance. If you get pulled over by the police (which may be likely if you are driving with US plates) then you may be up for a fine and other problems.

Perhaps one option is to have the vehicle towed to the garage to have the modifications made, which will then make it easier for you to obtain insurance.

I have not heard of ADAC but if they can offer some sort of solution, you should look into it.