Call a procedure with negative argument variable

rect := t -> Heaviside(t + 1/2) - Heaviside(t - 1/2)
intConv := (f1,f2) -> t -> (int(f1(t-tau)*f1(tau), tau = -infinity..infinity));

What I now wanna do is something like
plot(intConv(Heaviside, rect(-t)));
but it doesn't work and only
plot(intConv(Heaviside, rect));
works without giving arguments at all, even
plot(intConv(Heaviside(t), rect(t)));
doesn't work?