What is the model number? Every stereo amp that I have ever seen does wire differently for mono, if it can be used as a mono amp.
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More often than not you have to connect the speaker between the positive output terminals of each amplifier channel, and set a switch that is on the amp to mono mode. The switch, and the circuit it controls, cause one channel to be fed an inverted signal, relative to the other channel.
On some amps, though, there is no switch, and the positive terminal of one channel is common with the negative terminal of the other channel, which means that the phase inversion is always present. In normal stereo operation that would be corrected for by reversing the plus and minus speaker connections on one channel, and would be reflected in the red and black color coding of the speaker terminals. In mono mode, the speaker would be connected between the two non-common terminals, one positive (red) and one negative (black).