The plastic is old and has lost its 'plasticity'. You will probably have to use a screw driver to press in the area marked 'press', and pry them apart. Unfortunatly, it may break the tab, but that may be your only recourse. I have run into this problem many times servicing my 1995 automobile. over the life of the part, it is subjected to uv, and other oxidizing elements, and becomes brittle. I'm sorry I dont have any other suggestions for you.

Rick