Chaining mouse event handlers in BP7

I'm having the following problem:

I want to install a mouse eventhandler for string mouseclicks, but
there is already a handler for mousemoves (to take care of drawing of
the pointer in SVGA modes). With interrupt 33 function $14 i get the
adress of the already existing handler. Now i install my own handler
which at the end calls the old handler.

I'm not getting it right! does anyone has experience in this matter
and/or has a good (hopefully working) example in this area?

I'm succeeding in calling the old handler but it looks like my AX
register get's messed up. In P-mode it will crash the hole system.

Anco Scholte ter Horst.