An RS232 link cannot use just 2 wires; there must be at least RX, TX, and Ground - a minimum of three
Of course, the Ground could be shared with the power supply - do you have a common ground reference between the sensor and the Wavecom unit?
Note that, by default, Wavecom requires some of the hanshaking signals to be active; eg, see: viewtopic.php?f=53&t=3081&p=11377&hilit=loopback+rts+cts#p11377
You either need to provide those signals (eg, by loop-backs), or disable hardware flow control.