Clogged Drain Line

I’m having problems with water draining down from my freezer to my refrigerator. I suspected a clogged drain line. I powered down the refrigerator and melted all of the ice that had built up in the freezer. I cleaned out everything and tried to clean out the drain line with a piece of stiff wire. The wire only goes down the drain from the freezer about 3 inches and then hits something and won’t go any further. I removed the bottom back cover and put a wire up the drain line and it goes all the way up to about this same point. I tried hot water, I tried blowing out the blockage, all to no avail. I don’t have access to the drain line as it is behind the back cover and the cover can’t be removed without totally disassemblying the refrigerator. Any suggestions on what I might be hitting and how to remove the blockage?

It might be a check valve