13+ wire DB-25 or DB-15 cable
A DB-25 cable with only 6 wires is probably configured as a nullmodem. Try to find a straight EIA-232 cable for connection between DCE (Data Communication Equipment, i.e. modem) and DTE (Data Terminal Equipment, aka PC), this should have all 25 pins connected.  

A DB-15 VGA cable should also give you 13 wires, but return ground for red, blue and green may share the same wire (they shouldn't).