Did the fuse blow, out of the blue? In that case you might want to investigate why it possibly happened, maybe some wires had been chafing for a while then suddenly finally shorted.
If that happens in the future again or intermittently, then you will know that the car is suffering from deterioration of the insulation of certain wires and maybe eventually you will want them replaced. Track it down, you may find a section that needs a quick touch-up.
Electrical problems could be caused by the infamous `biodegradable` wiring harness since Volvo did not get to use the best materials for certain model years.
Great roadside fix, nice to hear that it got solved on the spot and before losing too much of the battery power! Yeah...the injection pump needs the constant 12Volts... this fact could be exploited as advantage: to build some simple, hidden security cut-off switch for our cars against thieves who happen to be interested in 30-40yo rustboxes with fading peeling paint and brittle plastics (sarcastic). An Immobiliser, an anti-theft device
but the 3 pedals already act as a form of deterrent.
)