The 7900, and most any contemporary radio is spec'd for 13.8V DC input, plus or minus 15%. It *should* put out rated power down to 11.73V. That means 11.73V at the back of the radio, not at the end of the power cord.
If we run the numbers:
T9021715 Yaesu Power Cord 14AWG, 9ft/Pair, 18ft/single
14AWG=.002524*18= .045432 ohmsTX@8.5A
= .386172 volt drop. Figure about 5 milliohms of internal impedance for a fully charged SLI battery, so .0425V of drop at the terminals.
.386172+.0425=.428672 volts of drop when transmitting on a 7900. The resting voltage of a car battery is nominally 12.78V, so 12.78-.428672=~12.35V.
That is plenty of headroom for a radio spec'd to 11.73V. With the engine running there's a good volt to volt and a half extra which could easily make up for a slightly resistive termination or fuseholder.