If you're looking for the definitive, detached explanation of what the war in El Salvador is all about, a kind of mix between "Roots" and "Apocalypse Now" with nonstop voice-over by Alistair Cooke, tonight's "Frontline" on Channel 26 at 8 is not the place to find it.

Instead, the voices in "Crossfire: El Salvador" are Salvadoran and the message is the kind that television does best: This is how it looks and sounds there now.

There is a blessed lack of politicians, with one exception, and not a lot of talk. It's mostly sights and sounds from the perspectives of the three major groups: the armed forces, the guerrillas and the peasants. But that is a great deal, and it is done grippingly. Look up the case history and the prognosis elsewhere; here are the symptoms of a nation in extremis.

The one-hour visit opens with a bloody body alongside a busy road, a sight that the locals convincingly tell us is an everyday event they hardly notice anymore. There is a riveting interview with foul-mouthed "Ricardo," who says he is a member of a paramilitary death squad and personally has killed 23 guerrillas.

Ricardo will probably be annoyed to find that his face is clearly visible for a few moments, although we are told we see him only in silhouette. How does he get information? "----, it's obvious, we torture them . . . same as you did in Vietnam. We learned from you," he says cheerfully.

The producers split up the politics. Jeff B. Harmon covered the army and the right wing, while Chris Wenner followed the guerrillas and the peasants, who mainly wish the fighting would stop. It seems to have been an effective approach, each producer focusing on the telling moments that carry punch.

For flavor, there are battle scenes from all three perspectives, the camera cowering in a bug-ridden bomb shelter with an old woman and a baby, jumping out of a helicopter with the army, slinking through the jungle with the guerrillas.

A machine-gun rattle becomes a guitar riff as we also see all three groups at rest. The guerrillas exercise and learn to read--propaganda, naturally--and watch television in their strongholds; the soldiers sing and play soccer. Both sides tell their own horror stories.

The obligatory poolside disco scene appears a cliche'--"There's no war here," laughs a sweaty dancer--until the camera focuses on the round stamp mark the patrons receive on their wrists at the entrance. It dissolves into the identical stamp mark on a corpse at the morgue, reminding us that this very recently was a human being enjoying himself.

Roberto D'Aubuisson, the rightist president of the constituent assembly, is the only politician in the show and turns the tables on the interviewer, asking why nobody ever asks him about the soldiers and officials murdered by the guerrillas. Then we hear little Javier, about 7, murmur the heartbreaking tale of how he survived the massacre of his family by hiding under the bodies. It's clear that who did it is the least of his worries now.

Yes, business is flourishing at La Protectora funeral home on Concepcion Street, where storekeepers insist they never noticed a thing last night, even though the blood of the body found there this morning is still in the gutter.

One of the most single-minded rightists in the country, Emilio Redaelli, describes the guerrillas' murder of his father in the hard-edged way so common in El Salvador, the manner that seems to make compromise a laughable dream. There is a Salvadoran saying that applies to his recommended solution, he tells us later: "Kill the dog and the rabies ends."

Unfortunately, that also seems to be the preferred solution of the other side in this conflict, which is what makes the situation so achingly difficult for everybody else.